WEATHER FORECASTING This problem has two parts . . . Analyze: What’s going on right now? Forecast: What’s going to happen?

Let’s start with a question. . . Why do we have weather? The Earth rotates on a tilted axis. Due to the tilt, the equator is always heated more than either pole.

The Result . . . Part of Earth’s atmosphere is too cold and dense and part of the atmosphere is much too warm and light Somehow . . . this must all be balanced out.

Cloud Formation (Cont’d) Warm air rises (less dense), and will gradually cool. The air will begin condensing (when the temperature and dewpoint become closer together), and water droplets bond onto condensation nuclei. These nuclei will collide with other nuclei, eventually forming a cloud. Air can reach saturation in a number of ways. The most common way is through lifting. As a bubble or parcel of air rises it moves into an area of lower pressure (pressure decreases with height). As this occurs the parcel expands. This requires energy, or work, which takes heat away from the parcel. So as air rises it cools. This is called an adiabatic process. Since cold air can hold less water vapor than warm air, some of the vapor will condense onto tiny clay and salt particles called condensation nuclei. The reverse is also true. As a parcel of air sinks it encounters increasing pressure so it is squeezed inward. This adds heat to the parcel so it warms as it sinks. Warm air can hold more water vapor than cold air, so clouds tend to evaporate as air sinks.

CLOUDS Cirrus Stratus Cumulus Nimbus

Cirrus Clouds High-level clouds Usually only ice crystals Generally in fair weather

Stratus Clouds Base is usually only a few hundred feet above the ground Little to no vertical development Can cover entire sky

Cumulus Clouds Base is at low level, but tops can reach 60,000 feet (11 miles) high Made of both ice and water droplets Puffy like cotton balls

Nimbus Clouds Generally form 7,000 to 15,000 feet (1 to 3 miles) above ground Steady precipitation

PRECIPITATION Two basic ways precipitation forms: “Collision” process (warm clouds) “Ice Crystal” process (cold clouds)

“Collision” Process

“Ice Crystal” Process Easier for water vapor to deposit directly onto ice crystals. Crystals then grow heavy enough to start falling.

Types of Fronts Now that we have learned the concepts of density, we can better understand what fronts are and how they affect the weather. Cold Front: A transition zone where cold/dry, and stable air, replaces warm/moist, and unstable air. It is depicted by a blue line with triangles pointing towards the direction of movement. Fronts are a dividing line that separates two different air masses. There are three different types of fronts: 1.) cold fronts 2.) warm fronts 3.) stationary fronts.

Cold Front Colder 27 Temperatures 54 33 Warmer Temperatures 62 Typically, bad weather (thunderstorms) is associated with cold fronts. This threat peaks especially when cold fronts intersect warm fronts near low pressure systems. 62

Cold Front (Vertical Profile) Advancing Cold air Rising Warm air Because cold air is denser than warm air, it can be best referred to as a deep dome of air, capable of easily lifting the warmer, less dense air, forming showers and thunderstorms through the collision and coalescence process.

Warm Front Colder 45 Temperatures Warm Front 38 55 Warmer Temperatures 62 Warm Front: A transition zone where warm, moist air replaces cold, dry air. Typically, warm fronts only travel at half the speed than that of cold fronts (because of the density principle). These fronts are depicted as the red line with semicircles pointing towards the direction of movement.

Warm Front (Vertical Profile) air Cold air Since warm air is less dense than cold air, this air mass can be best referred to as a shallow wedge of air, which struggles to lift the heavier/denser cold air. A good analogy can be a weightlifter trying to push a large dump truck…this individual will have trouble pushing the vehicle. Warm fronts do not slope in the vertical as much as cold fronts do. The result will be the warm air gently overriding the cooler, less dense air, thus only light to moderate showers will form at best. The rain/snow will fall ahead of the warm front. Light to moderate showers and a few thunderstorms will be possible on the north side of the warm front. This is due to the gentleness of the slope of this kind of air mass.

Stationary Front a pair of air masses, neither of which is strong enough to replace the other Cold Air Stationary Front: A boundary that has essentially no movement, but separates warm, moist air from cold, dry air. It is depicted by an alternating red and blue line containing blue triangles and red semicircles. The triangles point towards the warmer air, while the semicircles point towards the cooler air. Here, showers and thunderstorms can be more widespread/numerous on both sides of this front, as this precipitation continues to move parallel to this boundary over the same areas. This scenario can result in extensive flooding given the right conditions. Warm Air

Occluded Front a cold front overtakes a warm front

In order to form, thunderstorms need: Moisture Instability Lifting

INSTABILITY If air is stable, it will try to go back to where it was If air is unstable, it will continue in the direction it was pushed

LIFT Differences in heating Terrain Fronts, boundaries, drylines

The three stages in a thunderstorm’s life:

Thunderstorm Hazards Hail Damaging Winds Tornados Flash Floods Lightening

HAIL

DAMAGING WINDS Damage from a tornado Damage from a downburst

TORNADOS F-Scale Named after its creator, Dr. Fujita Used to describe how fast winds in a tornado are Actually goes all the way to F12, which is the speed of sound

Minimal Tornado - F0, F1 - 67% of S.C. TX Tornadoes - Causes 5% of all deaths - Life span 1 to 2 minutes - Path length less than 1 mile - Path width less than 100 yards - Wind speeds up to 110 mph

Strong Tornado - F2, F3 - 30% of S.C. TX Tornadoes - Causes 30% of all deaths Life span 15 to 20 minutes Path up to 15 miles - Path width up to 500 yards - Wind speeds up to 200 mph

Violent Tornado - F4, F5 - 3% of S.C. TX Tornadoes - Causes 65% of all deaths - Life span to several hours - Path length dozens of miles - Path width to 1 1/ 2 miles - Wind speeds over 300 mph